1. Field of the Invention
The present relates to line card protection and, more particularly, to an apparatus for protecting a line card during power cross situations.
2. Description of the Prior Art
Telephone line cards are connected to subscriber equipment by a pair of metallic wires called tip and ring lines. A series resistor of approximately 100 ohms is usually inserted in each of the lines to limit and adjust line current. During normal operation the lines are subjected to approximately 40-50 volts DC, which results, in conjunction with the series resistors and the normal loop resistance, in a current of 20-40 ma.
In many situations, the metallic telephone lines are strung on utility poles below power distribution lines and, on occasion, a live power line can accidentally fall across the metallic telephone lines, causing the line card to be subjected to high voltage levels which may range between 220 to over 1000 volts. This situation is known in the art as power cross, and it is essential that the line card be protected in such situations. The line card should preferably be protected from failure, and it is extremely important that the card not overheat to a level that could cause a fire.
Using a fuse in such a situation would appear to be the normal solution; however, it has been found that the resistance in the line limits the current to an extent that it is not high enough to cause a fuse to melt. Thus, in many instances, high current levels are provided to the line card circuit for an extended period of time, causing card failure and possible heat buildup and the risk of fire.
Heretofore, an effective, inexpensive solution to this potentially disastrous problem has not been provided.